The present invention relates to boosted brake systems for motor vehicles and more particularly to systems including more than one vacuum booster.
The elements of these systems are well known to an average person skilled in the art and neither their construction nor their functioning will be dealt with here.
The document U.S. Pat. No. 3,201,177 describes, for example, a system which includes a vacuum booster controlled by a pedal acting on a single master cylinder feeding the brake motors of the front wheels of a vehicle, and a second vacuum booster connected pneumatically to the first and acting on another single master cylinder feeding the brake motors of the rear wheels.
The document U.S. Pat. No. 4,071,283 also makes known a similar system, in which the second booster is fed via a solenoid valve determining the communication of one of its two chambers with the open air, thereby making it possible to perform functions, such as those of a pressure compensator or of a rear-wheel anti-lock system, simply by acting on this solenoid valve.
The document U.S. Pat. No. 4,708,401 describes an alternative version of such a system, in which the solenoid valve is a multi-directional pneumatic solenoid valve.
These systems suffer from many disadvantages. In fact, the reliability and lifetime of a solenoid valve are not identical to those of a vacuum booster, and this can give rise to the deactivation of the brakes of the rear wheels of the vehicle. Moreover, brake systems with three hydraulic circuits or more are considered unsuitable by motor-vehicle manufacturers because of the corresponding increase in the number of bleeding and filling operations to be carried out on the line.